1. Field of the Invention
The invention relates to precious metals, precious metal alloys, and jewelry made therefrom.
2. Prior Art
Jewelry pieces formed from precious metals that are white in color are highly desirable among jewelry consumers. There are many ways to achieve white pieces. However, if one requires that the piece be formed of precious metal, the options become much more limited. Precious metals include silver and gold plus the platinum group metals, namely, platinum, palladium, ruthenium, rhodium, osmium, and iridium.
One option for making a precious metal piece that is white in appearance is to make the piece from platinum. Platinum provides excellent luster, and it is ductile and malleable, making it a good material for making jewelry. Platinum is also highly resistant to corrosion and to wear, when alloyed to increase its hardness. However, platinum has significant drawbacks. Platinum has a very high melting point relative to gold: 3215° F. versus 1948° F. for gold. Molten platinum also cools and solidifies very quickly—on the order of about one second after pouring. The high melting point of platinum significantly effects the efficiency of using platinum in traditional jewelry processes, such as investment casting.
In investment casting, also called lost wax casting, a sprue or “tree” is formed of wax. Wax models of the items to be cast are mounted on each branch of the wax tree. Traditionally, beeswax was used, though today many different types of wax as well as plastics and even frozen liquid metals such as mercury are used. An investment mold is formed around the tree using plaster, ceramics, or other suitable materials. Once the mold has hardened, the wax is removed, typically by heating it and allowing it to melt and run out of the mold. This leaves a void in the mold the same shape as the tree. Molten precious metal may then be poured into the mold. The liquid metal will flow through the mold, filling the space left by the wax. In so doing, the liquid metal will fill the spaces left by the wax models, thereby forming a precious metal casting of the jewelry items. When the metal has hardened, the plaster mold is removed, leaving a precious metal “tree” that should be substantially identical to the wax tree present at the outset of the process. The pieces may then be removed from the tree while the branches and trunk may be reused.
It will be appreciated that the molten metal must remain liquid long enough to fill the tree. Thus, the rate at which a metal cools and solidifies determines how large a tree may be used. Stated differently, metals that cool and solidify quickly can only form a few pieces at a time via lost wax casting.
Because of the rapid cooling rate of platinum, special measures are taken to form platinum pieces via investment molding. The mold is heated to a high temperature, typically about 1500° F. The entire mold is rotated to use centripetal force to quickly pull the molten platinum into the peripheral cavities in the tree. The crucible is also rotated so that the molten platinum leaving the crucible will exit at a higher flow rate, again to promote more rapid filling of the mold.
To achieve an acceleration of the molten platinum flow rate from the rotation of the crucible, it will be appreciated that the outflow aperture should be on the side of the crucible, rather than at the top or the bottom. If the crucible is rotating about a top to bottom axis, for centripetal force to accelerate the flow rate of the molten platinum exiting the crucible, the outflow aperture must be on the periphery of the crucible, rather than on the axis. The outflow aperture, then, will rotate about the axis of rotation of the crucible. For the molten platinum to flow into the mold, the mold must remain aligned with the aperture. Accordingly, the mold is positioned at an approximate right angle to the axis of rotation of the crucible and the mold also revolves about the axis of rotation of the crucible, in synchronicity with the crucible. Thus, in platinum investment casting, the mold is typically positioned laterally relative to the crucible, rather than vertically beneath the crucible and the mold has two degrees of motion: the mold revolves around the crucible's axis of rotation and the mold rotates about its own axis. This rotational molding process is obviously much more complicated than static molding wherein molten metal is simply poured out of a crucible into a stationary, though often pre-heated, mold. Despite the additional effort, typical yields for rotational investment casting of most platinum alloys, with a pre-heated (˜1500° F.) mold, are no more than about five pieces per mold.
Another significant drawback to platinum is its price. Platinum is currently trading at about (US) $1725 per troy ounce, making it very expensive as a jewelry making material.
Another option for making white jewelry pieces is white gold. White gold refers to a group of gold alloys that typically comprise gold and nickel and/or palladium. The majority of the white gold alloy will be gold, which also makes white gold relatively expensive, as gold is currently trading at about (US) $1157 per troy ounce.
However, the fact that white gold is primarily gold, means that white gold alloys are generally corrosion resistant and adequately ductile and malleable for jewelry making purposes.
White gold is superior to platinum in casting efficiency. Depending upon the specific composition of the alloy, white gold will melt between about 1600° F. and 1800° F. Molten white gold also solidifies much more slowly than platinum, typically on the order of about five seconds, when poured into a static mold pre-heated to about 900° F. As a result, lost wax casting “trees” containing many more finished pieces may be obtained with white gold, than with platinum. Trees containing fifty to seventy-five white gold pieces are common.
In addition to cost, another significant drawback is that much white gold, despite its name, is not really white. Rather, it is to varying degrees, somewhat yellow. As a result, white gold pieces are commonly plated with rhodium. Rhodium gives a superb white finish, even compared to the highest quality white gold. However, the rhodium plating is subject to wear, which can be an especially significant issue with a high wear item like a ring. Any wear of the rhodium plating will almost inevitably be uneven. This will result in a contrast between the remaining rhodium plating and the underlying piece. This contrast will seldom be aesthetically pleasing and may be particularly jarring when the underlying piece has a high degree of yellow coloration.
Rhodium is very expensive, currently trading at about (US) $2780 per troy ounce. The use of white gold, to the extent that it involves rhodium plating, will increase the overall cost of the piece and add a step to the manufacturing process. Rhodium plating will also make the piece subject to wear, such that replating will often be required to maintain the appearance of white gold pieces over their lives.
Another problem with white gold is that much of it, especially in the United States, includes nickel. Nickel can be used to obtain an alloy that is sufficiently white so as to not require rhodium plating. However, nickel can also cause an allergic contact dermatitis in persons susceptible to the allergy and who come into contact with the metal. Thus, the presence of nickel in white gold poses an allergic risk to a subset of the population. Nickel can also increase the brittleness of the piece.
Still another option for white jewelry pieces is silver. One of the primary drawbacks of silver is its tendency to tarnish. When polished, silver has a highly lustrous, white appearance. However, after only a brief time in service, it will tarnish, such that frequent polishing is required to maintain the appearance of silver pieces.
Another consideration with silver is its weight. Though lighter than gold and platinum, (the density of gold is about 19.3 grams per cubic centimeter; platinum, 21.4 grams per cubic centimeter), silver has a nice weight at about 10.5 grams per cubic centimeter. By comparison, fourteen karat gold alloys have a density from about 12.9 to 14.6 grams per cubic centimeter, depending upon the make-up of the non-gold portions of the alloy. Likewise, ten karat gold alloys have a density of about 11.4 grams per cubic centimeter. Although pieces made of silver would feel light compared to pieces made of pure gold or platinum, the more relevant comparison is their feel compared to pieces formed of common jewelry making alloys. As illustrated above, silver is only about twenty percent less dense than fourteen karat gold and only about four percent less dense than ten karat gold. Thus, pieces made of silver will not feel markedly lighter than comparably sized pieces made of fourteen karat, or especially ten karat, gold.
Silver has a melting point of about 1763° F. Thus, its melting temperature is comparable with that of gold, less than two hundred degrees (F) separating the two. Like white gold, silver solidifies much more slowly than platinum—remaining liquid on the order of about five seconds when poured into a static mold pre-heated to about 800° to 1000° F.
An advantage of silver is its cost. Silver is currently trading at about (US) $18.00 per troy ounce. Thus, pieces made of silver will cost much less than comparable pieces made of gold or its alloys.
Another advantage of silver is its ductility and malleability. Silver is highly ductile and malleable, making it a good choice for forming jewelry.
Yet another option for white jewelry pieces is palladium. Palladium is whiter than platinum. Thus, unlike white gold, rhodium plating is not needed for palladium jewelry. Palladium has a melting point of about 2831° F., which is higher than gold but much lower than platinum. When molten, common palladium alloys also cool more slowly than platinum, typically solidifying in about five seconds after pouring into a static mold pre-heated to about 900° F., allowing more pieces to be cast using lost wax methods than with platinum. “Trees” including ten to twenty pieces can often be cast using palladium.
At 12.0 grams per cubic centimeter, palladium is more dense than silver, though less dense than gold or platinum. Palladium is very resistant to tarnishing under normal atmospheric conditions. It is also sufficiently ductile and malleable to be worked as jewelry.
A principle drawback of palladium, relative to silver, is its cost. Palladium currently trades at about (US) $515 per troy ounce. This makes it more affordable than gold or platinum, but relatively expensive compared to silver.
In view of the foregoing, precious metal alloys meeting the following objectives and jewelry manufactured therefrom are desired.